When etching an In series compound semiconductor using a halogen gas, the vapor pressure of the reaction product of In and the crystal and halogen gas is quite low. In contrast thereto, the vapor pressure of other reaction products and the halogen gas is high. For example, when InP is etched with chlorine, the vapor pressure of InCl.sub.3 is 10.sup.-8 Torr at 100.degree. C. and the vapor pressure of PCl.sub.3 is 57 Torr at 400.degree. C. Therefore, reaction products other than In compounds are likely to be selectively desorbed from the In series compound semiconductor, and there arise problems that surface roughness occurs after etching and a tapered etched configuration is produced. In order to solve these problems, physical sputtering with high energy ions having energies of several hundreds of eV to several thousands of eV employing RIBE (reactive ion beam etching) or RIE (reactive ion etching) at a substrate temperature above 200.degree. C. to promote desorption, has been examined. However, these methods involve a large impact by ions accelerated to a high energy, whereby the substrate is damaged.
The prior art etching of In series compound semiconductors is performed as described above. It was difficult to etch In series compound semiconductors to produce a good surface morphology and make the side surfaces of the etched concave portion vertical, thereby resulting in less damage.
The inventor of the present invention has invented an etching method for etching of In series compound semiconductors at low ion energy by employing an ECR etching apparatus and employing a Cl.sub.2 /He mixture which is obtained by diluting Cl.sub.2 gas with He gas more than four times to make the ratio of Cl.sub.2 /(He+Cl.sub.2) 0.05.about.0.2 and using a gas pressure below 4.0.times.10.sup.-4 Torr. By employing this etching method, it is possible to etch In series compound semiconductors with good surface morphology and low damage. In this method, however, there is a tapered etched side surface of the etched concave portion and it has been impossible to etch In series compound to produce an etched side surface having an etched concave portion that is vertical.